The compression-ignition internal combustion engine, or ‘diesel’ engine as it is more commonly known in the art, is a propulsion system that is used in many on-road and off-road applications, for example: small and large family cars, freight carrying vehicles, electrical power generation and marine propulsion systems.
As shown in FIG. 1, a typical diesel engine system 2 includes an engine block 4 and a fuel delivery system 6 for delivering fuel to the cylinders (not shown) of the engine block 4. The fuel delivery system 6 comprises a plurality of electronically-operated fuel injectors 8, one associated with each respective cylinder of the engine block 4. It should be appreciated that the diesel engine system 2 shown in FIG. 1 has been simplified for present purposes.
The fuel injectors 8 are supplied with high pressure fuel from a high pressure fuel accumulator volume 10, which is more usually referred to as a ‘common rail’. The common rail 10 is in the form of a metallic body that defines an internal volume for receiving and housing pressurised fuel. A fuel pump 12 draws low pressure fuel from a fuel tank 14, and supplies high pressure fuel to the common rail 10.
The volume of fuel that is delivered by the injectors 8 to the engine is controlled by an engine control system 16. The engine control system 16 receives, by way of a sensor input data link 18, real time data relating to many vehicle parameters such as engine speed, engine temperature and throttle pedal position and, in response to such sensor input, calculates an appropriate volume of fuel to deliver to the cylinders of the engine so as to achieve the desired operating condition.
The volume of fuel that is delivered by the injectors 8 is generally a function of the pressure of fuel and the time period for which the injector is ‘open’. It is therefore important for the pressure of fuel stored in the common rail 10 to be controlled precisely in order for the combustion process to be maintained at an optimum level.
There are certain considerations that govern the design of a common rail for any given application. For instance, in some engine applications the load on the engine changes abruptly. In order to maintain optimum combustion under such load changes it is desirable for the pressure of fuel within the common rail to be increased significantly and promptly when the engine load increases. In such circumstances it is preferable for the internal volume of the common rail to be kept relatively small. On the other hand, it is desirable for the pressure of fuel in the common rail to be unresponsive to injector filling events and a larger volume is more suitable for this purpose. However, in practice, each of these design constraints comes with disadvantages so the design of the common rail results in a compromise between providing a common rail with sufficient volume so that it is acceptably robust to unwanted pressure changes but with a small enough volume so that the high pressure fuel pump can change the fuel pressure in the common rail rapidly enough to maintain optimum combustion.
It is an object of the invention to provide an improved common rail that avoids or at least mitigates at least some of the aforementioned problems that are associated with existing high pressure common rail devices.